An air diffusion device of this type is known from German Patent DE-PS No. 29 42 607. In the known air diffusion device, the perforated air diffusion foil is joined to a tight backing by means of a closed frame. The tight backing comprises an unperforated foil, which is supported by a plurality of cross strips extending between the sides of the frame. Since the air diffusion device of the type described must be continuously under water during operation, the closed frame and the cross pieces extending between the sides of the frame must be made of nonrusting material. Manufacturing the closed frame and fastening the unperforated and perforated foils is very costly.